The present invention relates to a method and apparatus for protecting a press from being damaged by overload conditions, and more particularly to a method and apparatus which utilize a digital circuit to detect the angular velocity, angular displacement, and angular acceleration, and then utilize a digital information processing circuit to calculate a pressure load, torsion load, power load, and energy load based on the detected values. When at least one of the calculated loads exceeds its respective permitted safe limit, the digital information processing circuit will effect a braking of the press, and release hydraulic buffer means of the press to protect the press and die from suffering damage.
In the operation of a press, any overload conditions will result in the locking of the die and the ram of the press, which may cause the press to be distorted, to lose its precision, and to suffer damage. Therefore, several protecting ethods have been proposed in the art, and the following are the prevalent protecting methods;
(a) A hydraulic buffer device is provided within the ram of the press for relieving high pressure oil to reduce the impulse of the ram when the working pressure of the ram exceeds a pre-set pressure limit.
(b) A strain measuring device, such as a strain gauge or load cell, is mounted on the press to effect the braking of the press when the measured strain of the press exceeds a pre-set value.
(c) A buffer mechanism is mounted within the ram. When the working pressure of the ram exceeds a pre-set pressure limit, the buffer mechanism will be broken to protect the ram from locking into the die.
However, only when the pressure overload condition occurs can the protecting methods mentioned above stop the press. When suffering the other possible overload conditions, however, such as torsion overload, power overload, and energy overload, the press will still suffer damage. Thus, the method and apparatus which can protect the press from damage under pressure, torsion, power, and energy overload conditions is eagerlywished for.
With reference to FIG. 1, there is shown a press capacity curve A and three overload states. The ordinate represents the press capacity of the press, while the abscissa represnts the distance away from the lower dead center. From the press capacity curve A, the section from the origin to the distance X is the working stroke of maximum pre-set pressure of the press, and occupies only a small part of the entire working stroke.
A curve B represents the pressure overload curve. The pressure overload may occur, for example, when the degree of the hardness of the work piece is too great, or when the ram cannot press the work piece smoothly, with the result that the pressure suddenly increases and exceeds the maximum pre-set pressure.
A curve C represents the torsion overload curve. The torsion overload may occur, for example, when a hard wrench is inadvertently placed upon the die. In this case, if the wrench has a height, e.g. Y, and the height Y is larger than the distance X, at the distance Y away from the lower dead center, the allowed pressure value of the capacity curve A is not high enough to withstand the working pressure. Although the working pressure does not exceed the maximum pre-set pressure at this time, the torsion withstood by the crank shaft has exceeded the pressure the capacity curve A can provide, whereby resulting in the torsion overload condition.
In the drawing the curve D represents a power overload curve. The power overload condition may occur, for example, when a soft and large object, such as a wood block, is inadvertently placed on the die. In this case, if the height of the wood block is Z, and is larger than the distance X, at the distance Z since the wood block is soft, the wood block can be continuing compressed by the press pressure, and thus absorbs and stores the press energy therein. The area encompassed by the curve D is larger than the area encompassed by the curve B as shown in FIG. 1, that is to say, within the wood block, a greater amount of press energy is absorbed than the press can provide. To be more specific, the drive motor of the press can only provide an energy of fixed power to store in the fly wheel. If the amoutn of energy lost is one working stroke of the ram is greater than the amount of fixed energy the drive motor can provide, the power overload condition occurs, and results in the velocity of the crank shaft gradually decreasing until the crank shaft stops, and the ram is locked with the die.
In addition, if the drive motor cannot provide the fly wheel with sufficient energy due to some fault, the energy stored in the fly wheel will be lower than the desired amount. Consequently, there is insufficient press energy than is needed for one working stroke, and the energy overload condition occurs.
According to the above description, the problems confronting the conventional protecting methods are as follows:
(1) Since, at the moment when the torsion, power, and energy overload conditions occurs, the pressure does not exceed the maximum pre-set pressure, the conventional protecting methods cannot effect the protecting process. As a result, the press will still be damaged under these overload conditions.
(2) In the conventional protecting methods, the oil pressure in the hydraulic buffer device must be lower than the maximum working pressure, resulting in the weakening of the rigidity of the ram structure. Consequently, the press error increases, and the punch-through phenomenon, which will damage the die, may occur.
(3) In the structure strain measuring method, a different measuring point effects the measuring result, above all, in the C-frame press.
(4) In the mechanism measuring method, repair is difficult.